1. Technical Field
This invention relates generally to a method for preventing compressor surge in a turbocharged Diesel engine and more particularly to such a method for controlling intake airflow during periods of temporary operation in a stoichiometric or richer combustion mode.
2. Background Art
The Environmental Protection Agency (EPA) has set very stringent emissions standards for heavy-duty vehicles that would reduce smog-causing emissions from trucks, buses and motor homes. The emissions standards set forth, which are to be fully implemented for model year 2010 mandate new, very stringent emission standards, as follows:                Particulate matter (PM)—0.01 g/bhp-hr        Nitrogen oxide (NOx)—0.20 g/bhp-hr        Non-methane hydrocarbons (NMHC)—0.14 g/bhp-hr.The particulate matter emissions standard will take full effect in the 2007 heavy-duty engine model year. The NOx and NMHC standards will be phased in for Diesel engines between 2007 and 2010. The phase-in would be on a percent-of-sales basis:50% in 2007–2009, and 100% in 2010.        
One of the most promising technologies for NOx treatment are NOx adsorbers, also known as “lean NOx traps.” Lean NOx traps need to be regenerated periodically, for example, up to one generation cycle every 30 seconds, to restore their efficiencies. The regeneration of lean NOx traps is usually done by providing reductants, such as CO and HC under oxygen-free conditions. Historically, lean burn engines, such as Diesel engines, have used exhaust-side supplemental fuel injection systems to reduce excess oxygen upstream of the lean NOx traps. From an efficiency standpoint, the supplemental fuel is wasted because it does not contribute to engine output power.
To avoid the efficiency penalty of supplemental fuel injection, several in-cylinder, low-smoke, stoichiometric combustion technologies have been proposed by which intake airflow through the engine is substantially reduced, generally by throttling the intake airflow. However, throttling of intake airflow can produce severe engine airflow disturbances, such as compressor surge, that propagate into the engine intake and exhaust manifolds and turbo machinery. Compressor surge is an unstable operating condition in which large mass airflow oscillations occur, and not only create adversely high noise levels, but can also damage various components of the turbocharger. Most compressors have a stability limit that is defined by a minimum flow rate on a pressure-rise-versus-flow-rate characteristic curve, commonly referred to as the surge limit or surge line.
Various methods have been proposed for controlling operation of the compressor stage of a turbocharged Diesel engine. For example, U.S. Pat. No. 6,295,816 granted Oct. 2, 2001 to Gallagher, et al., titled TURBO-CHARGED ENGINE COMBUSTION CHAMBER PRESSURE PROTECTION APPARATUS AND METHOD, describes a system in which a pressure relief valve in the compressor outlet is used to control peak pressure in the combustion chambers of the engine.
U.S. Pat. No. 6,564,784 granted May 20, 2003 to Onodera, et al. for an EXHAUST GAS RECIRCULATION CONTROL APPARATUS FOR INTERNAL COMBUSTION ENGINE; U.S. Pat. No. 6,701,710 granted Mar. 9, 2004 to Ahrens, et al. for a TURBOCHARGED ENGINE WITH TURBOCHARGER COMPRESSOR RECIRCULATION VALVE; and U.S. Pat. No. 5,526,645 granted Jun. 18, 1996 to Robert M. Kaiser for a DUAL-FUEL AND SPARK IGNITED GAS INTERNAL COMBUSTION ENGINE EXCESS AIR CONTROL SYSTEM AND METHOD, all describe methods by which boost air, i.e., compressed air discharged from the compressor stage of the turbocharger, is recirculated. More specifically, Onodera, et al. controls the exhaust gas recirculation flow rate by passing compressed air from the compressor outlet directly to the turbine inlet of the turbocharger system. Compressor discharge airflow is based on the airflow pressure differential across the engine. Ahrens, et al. similarly controls the airflow pressure differential across the engine to control the exhaust gas recirculation rate by passing boost air back into the compressor inlet. Similarly, Kaiser controls the airflow pressure differential across the engine by passing boost air back into the compressor inlet stage as a means of controlling intake manifold pressure.
However, none of the above-cited references describe a method for controlling intake airflow and compressor surge during temporary periods of stoichiometric or richer combustion mode operation during which exhaust gas aftertreatment devices are regenerated. The present invention is directed to overcoming such problems. It is desirable to have a method by which turbocharger boost pressure can be controlled to avoid compressor surge, particularly during periods of reduced airflow operation in a stoichiometric or richer combustion mode for the regeneration of a lean NOx trap or other regenerable exhaust gas aftertreatment device.